Pickling acids, such as hydrochloric acid and nitric acid, are used in the treatment of various metals. It is common practice to regenerate the acid solutions for reuse in the pickling process.
Pickling acids of hydrochloric acid produce numerous metal chlorides in the spent acid liquor. Many metal chlorides can be thermally converted into metal oxides and hydrogen chloride. The metal oxides then can be separated from the gaseous hydrogen chloride. The hydrogen chloride can be recovered as hydrochloric acid for regenerating the pickling acids.
Various processes are known for regenerating pickling acids. One process for regenerating pickling acids utilizes a roasting technique wherein the spent pickling acid solution from pickling iron or steel is sprayed into a furnace. The hot combustion gases in the furnace vaporize the spent pickling acid and oxidize the metal compounds to form metal oxide particles and hydrogen chloride. Other methods introduce the spent pickling acid into a fluidized bed. The pickling acid can be introduced directly into the fluidized bed. Alternatively, the metal chloride can be separated from the acid solution and then introduced to the fluidized bed as a slurry. One example of a fluidized bed process is disclosed in U.S. Pat. No. 4,049,788.
Fluidized bed and spray roasting processes are generally effective in the regeneration of pickling acids. However, these processes have some disadvantages that are difficult to overcome. For example, fluidized bed and spray roasting processes produce large amounts of fine particles in the gas stream which is very difficult to remove. The exhaust gas stream usually contains large amounts of fine particles of the metal oxides that are discharged to the atmosphere.
Efforts to limit the amount of fine particles of metal oxides discharged into the atmosphere have met with limited success. Various processes have been proposed for removing the fine particles from the gas stream. For example, cyclone separators and venturi washing devices have been used with some success. However, the exhaust gas still contains unacceptable amounts of fine particles. Accordingly, a continuing need exists in the industry for improved processes for regenerating pickling acids.